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Regulating Water-Salt Balance Kidneys keep the water-salt balance of the blood within normal limits. This allows for maintenance of blood volume and blood pressure.

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Reabsorption of Water When blood plasma becomes concentrated ( solute water), we say the osmotic pressure increases. Osmotic pressure: the force generated as water moves by osmosis (affects the exchange of materials between cells and blood) Osmoreceptors are cells that are sensitive to osmotic pressure. Most osmoreceptors are located in the hypothalamus.

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Hypertonic Blood – High Osmotic Pressure The osmoreceptors in the hypothalamus activate the pituitary gland to release antidiuretic hormone (ADH). ADH travels through the blood to the kidneys, where it increases the permeability of the Distal Tubule and collecting duct to allow more water to be reabsorbed into the blood. ▫This dilutes the blood and lowers osmotic pressure. ▫More concentrated urine is released.

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Hypotonic Blood – Low Osmotic Pressure When the blood plasma is too dilute: ▫Osmoreceptors in the hypothalamus stop the release of ADH. ▫The DT and collecting duct become less permeable to water. ▫This concentrates the solutes in the blood, causing osmotic pressure to increase. ▫More water is excreted in the urine and less concentrated urine is released.

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ADH – Negative Feedback

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Diuretics Increase the volume of urine. Stimulate urine production partly by inhibiting ADH release, which leads to increased urination and dehydration. Examples: caffeine and alcohol

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Diabetes Insipidus ADH activity is insufficient so a person urinates excessively. Water is excreted more quickly than it is consumed, resulting in: ▫Severe dehydration ▫Ion imbalances Synthetic ADH can be taken to restore the balance of water reabsorption.

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Reabsorption of Salts When blood volume is low, the kidneys secrete an enzyme called renin to trigger the release of the hormone aldosterone from the adrenal cortex. Aldosterone stimulates the excretion of K + and the reabsorption of Na + in the Distal Tubule and collecting duct. ▫This causes the passive reabsorption of Cl - and water, causing blood pressure and blood volume to increase.

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2. Respiration If the H + concentration rises in the blood, the respiratory centre in the medulla oblongata increases breathing rate. Increasing BR rids the body of H + through this reaction that takes place in the lung’s capillaries: An increased breathing rate pulls the reaction to the right to generate CO 2 more quickly. When more CO 2 is expelled, the number of H + decreases.

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3. Kidney Function The kidneys provide a slower acting but more powerful mechanism to adjust blood pH. They can excrete H + and reabsorb HCO 3 - as required. Ammonia (NH 3 ) produced in the tubule cells from the breakdown of amino acids can also remove excess H + in the urine. In the kidneys, bicarbonate ions (HCO 3 -) are reabsorbed and hydrogen ions (H + ) are excreted as needed to maintain the pH of the blood. Excess hydrogen ions are buffered, for example, by ammonia (NH 3 ), which is produced in tubule cells by the breakdown of amino acids. NH 3 + H +  NH 4

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Evaluating Kidney Function BLOOD TESTS can be used to analyze kidney function. ▫High amounts of urea and creatinine (a metabolic waste produced by muscles) in the blood indicate poor kidney function as these substances are normally filtered from the blood.

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Urinary Tract Infections More common in females than males due to the proximity of the urethra and anus in females. Symptoms include: ▫Burning sensation during urination ▫A frequent need to urinate ▫Bloody or brown urine Treatment: antibiotics Preventative measures include: proper hygiene and hydration. UTIs can become more serious if they reach the kidneys as kidney failure could occur

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Kidney Stones The development of crystalline formations in the kidney. Most develop because of excess calcium in the urine. The presence of calcium compounds may be due to urinary tract infections, chronic dehydration or low activity levels. Treatment depends on the size of the stones. Kidney stones are hard granules that can form in the renal pelvis. They are usually made of calcium, phosphate, uric acid, and protein. It is estimated that 5 to 15 percent of people will develop kidney stones at some time in their lives.

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Renal Insufficiency The state in which the kidneys cannot maintain homeostasis due to nephron damage. If 75% or more of the nephrons are destroyed, homeostasis can no longer be maintained and dialysis treatment or a kidney transplant is required. Dialysis: ▫The diffusion of dissolved substances through a semi-permeable membrane.